Oxidative stress is caused by disturbances to the normal redox state within cells. An imbalance between routine production and detoxification of reactive oxygen species such as peroxides and free radicals can result in oxidative damage to the cellular structure and machinery. The most important source of reactive oxygen species under normal conditions in aerobic organisms is probably the leakage of activated oxygen from mitochondria during normal oxidative respiration. Impairments associated with this process are suspected to contribute to mitochondrial disease, neurodegenerative disease, and diseases of aging.
Mitochondria are organelles in eukaryotic cells, popularly referred to as the “powerhouse” of the cell. One of their primary functions is oxidative phosphorylation. The molecule adenosine triphosphate (ATP) functions as an energy “currency” or energy carrier in the cell, and eukaryotic cells derive the majority of their ATP from biochemical processes carried out by mitochondria. These biochemical processes include the citric acid cycle (the tricarboxylic acid cycle, or Krebs cycle), which generates reduced nicotinamide adenine dinucleotide (NADH+H+) from oxidized nicotinamide adenine dinucleotide (NAD+), and oxidative phosphorylation, during which NADH+H+ is oxidized back to NAD+. (The citric acid cycle also reduces flavin adenine dinucleotide, or FAD, to FADH2; FADH2 also participates in oxidative phosphorylation.)
The electrons released by oxidation of NADH+H+ are shuttled down a series of protein complexes (Complex I, Complex II, Complex III, and Complex IV) known as the mitochondrial respiratory chain. These complexes are embedded in the inner membrane of the mitochondrion. Complex IV, at the end of the chain, transfers the electrons to oxygen, which is reduced to water. The energy released as these electrons traverse the complexes is used to generate a proton gradient across the inner membrane of the mitochondrion, which creates an electrochemical potential across the inner membrane. Another protein complex, Complex V (which is not directly associated with Complexes I, II, III and IV) uses the energy stored by the electrochemical gradient to convert ADP into ATP.
When cells in an organism are temporarily deprived of oxygen, anaerobic respiration is utilized until oxygen again becomes available or the cell dies. The pyruvate generated during glycolysis is converted to lactate during anaerobic respiration. The buildup of lactic acid is believed to be responsible for muscle fatigue during intense periods of activity, when oxygen cannot be supplied to the muscle cells. When oxygen again becomes available, the lactate is converted back into pyruvate for use in oxidative phosphorylation.
Oxygen poisoning or toxicity is caused by high concentrations of oxygen that may be damaging to the body and increase the formation of free-radicals and other structures such as nitric oxide, peroxynitrite, and trioxidane. Normally, the body has many defense systems against such damage but at higher concentrations of free oxygen, these systems are eventually overwhelmed with time, and the rate of damage to cell membranes exceeds the capacity of systems which control or repair it. Cell damage and cell death then results.
Qualitative and/or quantitative disruptions in the transport of oxygen to tissues result in energy disruption in the function of red cells and contribute to various diseases such as haemoglobinopathies. Haemoglobinopathy is a kind of genetic defect that results in abnormal structure of one of the globin chains of the hemoglobin molecule. Common haemoglobinopathies include thalassemia and sickle-cell disease. Thalassemia is an inherited autosomal recessive blood disease. In thalassemia, the genetic defect results in reduced rate of synthesis of one of the globin chains that make up hemoglobin. While thalassemia is a quantitative problem of too few globins synthesized, sickle-cell disease is a qualitative problem of synthesis of an incorrectly functioning globin. Sickle-cell disease is a blood disorder characterized by red blood cells that assume an abnormal, rigid, sickle shape. Sickling decreases the cells' flexibility and results in their restricted movement through blood vessels, depriving downstream tissues of oxygen.
Mitochondrial dysfunction contributes to various disease states. Some mitochondrial diseases are due to mutations or deletions in the mitochondrial genome. If a threshold proportion of mitochondria in the cell is defective, and if a threshold proportion of such cells within a tissue have defective mitochondria, symptoms of tissue or organ dysfunction can result. Practically any tissue can be affected, and a large variety of symptoms may be present, depending on the extent to which different tissues are involved. Some examples of mitochondrial diseases are Friedreich's ataxia (FRDA), Leber's Hereditary Optic Neuropathy (LHON), mitochondrial myopathy, encephalopathy, lactacidosis, and stroke (MELAS), Myoclonus Epilepsy Associated with Ragged-Red Fibers (MERRF) syndrome, Leigh's disease, and respiratory chain disorders. Most mitochondrial diseases involve children who manifest the signs and symptoms of accelerated aging, including neurodegenerative diseases, stroke, blindness, hearing impairment, vision impairment, diabetes, and heart failure.
Friedreich's ataxia is an autosomal recessive neurodegenerative and cardiodegenerative disorder caused by decreased levels of the protein Frataxin. The disease causes the progressive loss of voluntary motor coordination (ataxia) and cardiac complications. Symptoms typically begin in childhood, and the disease progressively worsens as the patient grows older; patients eventually become wheelchair-bound due to motor disabilities.
Leber's Hereditary Optic Neuropathy (LHON) is a disease characterized by blindness which occurs on average between 27 and 34 years of age. Other symptoms may also occur, such as cardiac abnormalities and neurological complications.
Mitochondrial myopathy, encephalopathy, lactacidosis, and stroke (MELAS) can manifest itself in infants, children, or young adults. Strokes, accompanied by vomiting and seizures, are one of the most serious symptoms; it is postulated that the metabolic impairment of mitochondria in certain areas of the brain is responsible for cell death and neurological lesions, rather than the impairment of blood flow as occurs in ischemic stroke.
Myoclonus Epilepsy Associated with Ragged-Red Fibers (MERRF) syndrome is one of a group of rare muscular disorders that are called mitochondrial encephalomyopathies. Mitochondrial encephalomyopathies are disorders in which a defect in the genetic material arises from a part of the cell structure that releases energy (mitochondria). This can cause a dysfunction of the brain and muscles (encephalomyopathies). The mitochondrial defect as well as “ragged-red fibers” (an abnormality of tissue when viewed under a microscope) are always present. The most characteristic symptom of MERRF syndrome is myoclonic seizures that are usually sudden, brief, jerking, spasms that can affect the limbs or the entire body, difficulty speaking (dysarthria), optic atrophy, short stature, hearing loss, dementia, and involuntary jerking of the eyes (nystagmus) may also occur.
Leigh's disease is a rare inherited neurometabolic disorder characterized by degeneration of the central nervous system where the symptoms usually begin between the ages of 3 months to 2 years and progress rapidly. In most children, the first signs may be poor sucking ability and loss of head control and motor skills. These symptoms may be accompanied by loss of appetite, vomiting, irritability, continuous crying, and seizures. As the disorder progresses, symptoms may also include generalized weakness, lack of muscle tone, and episodes of lactic acidosis, which can lead to impairment of respiratory and kidney function. Heart problems may also occur.
Co-Enzyme Q10 Deficiency is a respiratory chain disorder, with syndromes such as myopathy with exercise intolerance and recurrent myoglobin in the urine manifested by ataxia, seizures or mental retardation and leading to renal failure (Di Mauro et al., (2005) Neuromusc. Disord., 15:311-315), childhood-onset cerebellar ataxia and cerebellar atrophy (Masumeci et al., (2001) Neurology 56:849-855 and Lamperti et al., (2003) 60:1206:1208); and infantile encephalomyopathy associated with nephrosis. Biochemical measurement of muscle homogenates of patients with CoQ10 deficiency showed severely decreased activities of respiratory chain complexes I and II+III, while complex IV (COX) was moderately decreased (Gempel et al., (2007) Brain, 130(8):2037-2044).
Complex I Deficiency or NADH dehydrogenase NADH-CoQ reductase deficiency is a respiratory chain disorder, with symptoms classified by three major forms: (1) fatal infantile multisystem disorder, characterized by developmental delay, muscle weakness, heart disease, congenital lactic acidosis, and respiratory failure; (2) myopathy beginning in childhood or in adult life, manifesting as exercise intolerance or weakness; and (3) mitochondrial encephalomyopathy (including MELAS), which may begin in childhood or adult life and consists of variable combinations of symptoms and signs, including ophthalmoplegia, seizures, dementia, ataxia, hearing loss, pigmentary retinopathy, sensory neuropathy, and uncontrollable movements.
Complex II Deficiency or Succinate dehydrogenase deficiency is a respiratory chain disorder with symptoms including encephalomyopathy and various manifestations, including failure to thrive, developmental delay, hypotonia, lethargy, respiratory failure, ataxia, myoclonus and lactic acidosis.
Complex III Deficiency or Ubiquinone-cytochrome C oxidoreductase deficiency is a respiratory chain disorder with symptoms categorized in four major forms: (1) fatal infantile encephalomyopathy, congenital lactic acidosis, hypotonia, dystrophic posturing, seizures, and coma; (2) encephalomyopathies of later onset (childhood to adult life): various combinations of weakness, short stature, ataxia, dementia, hearing loss, sensory neuropathy, pigmentary retinopathy, and pyramidal signs; (3) myopathy, with exercise intolerance evolving into fixed weakness; and (4) infantile histiocytoid cardiomyopathy.
Complex IV Deficiency or Cytochrome C oxidase deficiency is a respiratory chain disorder with symptoms categorized in two major forms: (1) encephalomyopathy, where patients typically are normal for the first 6 to 12 months of life and then show developmental regression, ataxia, lactic acidosis, optic atrophy, ophthalmoplegia, nystagmus, dystonia, pyramidal signs, respiratory problems and frequent seizures; and (2) myopathy with two main variants: (a) Fatal infantile myopathy—may begin soon after birth and accompanied by hypotonia, weakness, lactic acidosis, ragged-red fibers, respiratory failure, and kidney problems: and (b) Benign infantile myopathy—may begin soon after birth and accompanied by hypotonia, weakness, lactic acidosis, ragged-red fibers, respiratory problems, but (if the child survives) followed by spontaneous improvement.
Complex V Deficiency or ATP synthase deficiency is a respiratory chain disorder including symptoms such as slow, progressive myopathy.
CPEO or Chronic Progressive External Ophthalmoplegia Syndrome is a respiratory chain disorder including symptoms such as visual myopathy, retinitis pigmentosa, or dysfunction of the central nervous system.
Kearns-Sayre Syndrome (KSS) is a mitochondrial disease characterized by a triad of features including: (1) typical onset in persons younger than age 20 years; (2) chronic, progressive, external ophthalmoplegia; and (3) pigmentary degeneration of the retina. In addition, KSS may include cardiac conduction defects, cerebellar ataxia, and raised cerebrospinal fluid (CSF) protein levels (e.g., >100 mg/dL). Additional features associated with KSS may include myopathy, dystonia, endocrine abnormalities (e.g., diabetes, growth retardation or short stature, and hypoparathyroidism), bilateral sensorineural deafness, dementia, cataracts, and proximal renal tubular acidosis.
In addition to congenital disorders involving inherited defective mitochondria, acquired mitochondrial dysfunction contributes to diseases, particularly neurodegenerative disorders associated with aging like Parkinson's, Alzheimer's, and Huntington's Diseases. The incidence of somatic mutations in mitochondrial DNA rises exponentially with age; diminished respiratory chain activity is found universally in aging people. Mitochondrial dysfunction is also implicated in excitoxic, neuronal injury, such as that associated with cerebral vascular accidents, seizures and ischemia.
The diseases above appear to be caused by defects in Complex I of the respiratory chain. Electron transfer from Complex I to the remainder of the respiratory chain is mediated by the compound coenzyme Q (also known as Ubiquinone). Oxidized coenzyme Q (CoQox or Ubiquinone) is reduced by Complex I to reduced coenzyme Q (CoQred or Ubiquinol). The reduced coenzyme Q then transfers its electrons to Complex III of the respiratory chain (skipping over complex II), where it is re-oxidized to CoQox (Ubiquinone). CoQox can then participate in further iterations of electron transfer.
Very few treatments are available for patients suffering from these mitochondrial diseases. Recently, the compound Idebenone has been proposed for treatment of Friedreich's ataxia. While the clinical effects of Idebenone have been relatively modest, the complications of mitochondrial diseases can be so severe that even marginally useful therapies are preferable to the untreated course of the disease. Another compound, MitoQ, has been proposed for treating mitochondrial disorders (see U.S. Pat. No. 7,179,928); clinical results for MitoQ have not yet been reported. Administration of coenzyme Q10 (CoQ10) and vitamin supplements has shown only transient beneficial effects in individual cases of KSS. CoQ10 supplementation has also been used for the treatment of CoQ10 deficiency with mixed results.
Oxidative stress is suspected to be important in neurodegenerative diseases such as Motor Neuron Disease, Amyotrophic Lateral Sclerosis (ALS), Creutzfeldt-Jakob disease, Machado-Joseph disease, Spino-cerebellar ataxia, Multiple sclerosis (MS), Parkinson's disease, Alzheimer's disease, and Huntington's disease. Oxidative stress is thought to be linked to certain cardiovascular disease and also plays a role in the ischemic cascade due to oxygen reperfusion injury following hypoxia. This cascade includes both strokes and heart attacks.
Damage accumulation theory, also known as the free radical theory of aging, invokes random effects of free radicals produced during aerobic metabolism that cause damage to DNA, lipids and proteins and accumulate over time. The concept of free radicals playing a role in the aging process was first introduced by Himan D (1956), Aging—A theory based on free-radical and radiation chemistry J. Gerontol. 11, 298-300.
According to the free radical theory of aging, the process of aging begins with oxygen metabolism (Valko et al, (2004) Role of oxygen radicals in DNA damage and cancer incidence, Mol. Cell. Biochem., 266, 37-56). Even under ideal conditions some electrons “leak” from the electron transport chain. These leaking electrons interact with oxygen to produce superoxide radicals, so that under physiological conditions, about 1-3% of the oxygen molecules in the mitochondria are converted into superoxide. The primary site of radical oxygen damage from superoxide radical is mitochondrial DNA (mtDNA) (Cadenas et al., (2000) Mitochondrial free radical generation, oxidative stress and aging, Free Radic. Res, 28, 601-609). The cell repairs much of the damage done to nuclear DNA (nDNA) but mtDNA cannot be fixed. Therefore, extensive mtDNA damage accumulates over time and shuts down mitochondria causing cells to die and the organism to age.
Some of the diseases associated with increasing age are cancer, diabetes mellitus, hypertension, atherosclerosis, ischemia/reperfusion injury, rheumatoid arthritis, neurodegenerative disorders such as dementia, Alzheimer's and Parkinson's. Diseases resulting from the process of aging as a physiological decline include decreases in muscle strength, cardiopulmonary function, vision and hearing as well as wrinkled skin and graying hair.
The ability to adjust biological production of energy has applications beyond the diseases described above. Various other disorders can result in suboptimal levels of energy biomarkers (sometimes also referred to as indicators of energetic function), such as ATP levels. Treatments for these disorders are also needed, in order to modulate one or more energy biomarkers to improve the health of the patient. In other applications, it can be desirable to modulate certain energy biomarkers away from their normal values in an individual that is not suffering from disease. For example, if an individual is undergoing an extremely strenuous undertaking, it can be desirable to raise the level of ATP in that individual.